I "think"? maybe what you saw is the cervid (deer type critter) removing the velvet from their antlers.
Each year when they grow they are encased in a fuzzy blood filled skin like covering refferred to as "velvet" because of its look and texture that the horns develop in.
For whitetails around here about late August/September they begin to do this to expose the underlying grown/hardened antlers.
Here are a few pics of white tails in velvet...

These two are a good example to compare growth also....
A trailcam pic...taken in May

Same pair Date/Time: 2007:07:26 08:24:40

Left one first pic...

Right one first pic...you can just see that cyst forming in first pic on it's left side.

A few others while in velvet...

Here is a moose in velvet.
You can see it is beginning to dry up inside by the marble look on the backside...

Here is an article I Googled up.
In it you can see what the trees they use look like...
I have never heard of them using rocks though.

We don't have Muleys here in Nova Scotia but all I could find is that the

does don't flag thier tails when alarmed as do whitetails
When both sexes of Muleys run the land on all four feet at once (that must look funny)
Mulyes are bigger in size than whitetails
Have huge ears like mules

kpr what is on that poor animals leg and why does it's horn droop? Was is in a zoo for health reason? Perhaps it should have been.

That white tailed deer is impressive

Mate...

The sores are caused by mosquito/blackfly bites...apparently it is not an issue for them.
The broken moose tine on its left antler...this sometimes happens when the antlers are in velvet.
I'm sure it causes some discomfort when it happens but the horn growing inside is not completely hard, more like cartiledge.
If you look back at page two there is a deer that looks to have a hole in the tip of his antler (right side I believe?) This was probably caused by a stick poking in there while still in velvet.
Again not an issue, antlers drop off every year and new bigger ones will grow next year.

RE:"in a zoo"

NONE of my critters are in zoos or captivity...all wild n free where they should be.

This one has an awful Fibroma on it's face.
They lessen in size in winter and it must be huge in summer (Grapefruit size at least)
Apparently not an issue though this one had two fawns/younguns with her.

Here is another with Fibroma's.
Not near the extent as the one pic'd above.
This one had them all summer but in Sept they "appeared" to clear up some.
This taken early Aug 07...

These late Sept 07

Same deer this year...see dark spot in left ear.

This particular deer I found very approachable.
I spent alot of time and patience with her walking along as she fed and talking to her.
After a while I could enter this field and almost call her over.
She would cross halfway across the field I the other half.
I was astounded when she did this the following year I'm looking fwd to seeing her this year, it'll floor me if it still happens.